/** The key of the message: Handler Name */
    public static final String LABELS_handler_name = "{labels.handler_name}";

    /** The key of the message: Parameter */
    public static final String LABELS_handler_parameter = "{labels.handler_parameter}";

    /** The key of the message: Script */
    public static final String LABELS_handler_script = "{labels.handler_script}";

       *
       * ...except that it's possible for users to insert a string key with a non-string value, and
       * in that case, getProperty *will* return null.
       *
       * TODO(b/192002623): Handle that case: Either:
       *
       * - Skip non-string keys and values entirely, as proposed in the linked bug.
       *
       * - Throw ClassCastException instead of NullPointerException, as documented in the current